To toggle the voltage supply on pin 12 you'll have to place a jumper on R29 or R31, both of which can be found to the right, and a little bit above the keypad header. By placing a 0 ohm resistor/jumper on R31, pin 12 supplies 5V. You can toggle pin 12 to GND by moving the jumper to R29. Keep in mind, only one jumper pad can be occupied at a time. If R31 and R29 are occupied, you will create a short.